The invention relates to an apparatus for inspecting a substantially hollow cylindrical region of transparent hollow bodies travelling along a track, whereby the bodies pass through an inspection zone on a section of the track, with an imaging device located on one side of the track including the inspection region, and a light source containing an illumination device located on the opposite side of the track and illuminating the inspection region.
A known device of this type (U.S. Pat. No. 4,691,231) is used for illuminating the sidewalls of bottles which are located on a continuously moving conveyor and are inspected by transmitted light, in order to detect defects in the walls of the bottles. For this purpose, the known device employs six video cameras for the side walls, three more video cameras for a lower side wall region and three video cameras for a side wall region above this. The video cameras may either be arranged in the shape of an arc around the bottle or in a line along the bottle conveyor. The cameras are arranged to provide each time data relating to regions of different height, over the whole circumference of the bottles. To achieve this, diametrically opposed bottle wall regions (front and back sides) are illuminated simultaneously and superimposed on each other. For each pair of cameras arranged one above the other a light source is provided at the side of the bottle opposite the camera; for a total of six cameras there are therefore three light sources. A bottle to be inspected is moved between the first pair of cameras and its corresponding light source and is thus recorded, then between the second pair of cameras and their corresponding light source, and finally between the third pair of cameras and their corresponding light source. A diffusing, transparent plate is disposed between the cameras and the light sources, so that the bottle is more or less equally illuminated for each pair of cameras.
For this known apparatus the illumination comprises illumination purely by transmitted light, for which the rear side and the front side (with respect to the camera) have the same effect on the path of the light beam. Such illumination is not suitable for examining one of the special regions of the bottle, for example the mouth region which is usually provided with a thread, when this is facing the imaging device, since this type of illumination does not give sufficient or uniform contrast to make faults or geometric structures in the glass, e.g. the thread visible. Such a region requires intensive high-contrast and uniform illumination to form the image necessary for inspection, in order that both internal and external defects, such as deposits or changes in the geometry of the glass, can be detected in this region, where the glass surface is very uneven due to the thread. Illumination purely by transmitted light is also unsuitable for inspection if the bottle is filled up to the mouth region with a substantially opaque liquid, or the bottle glass itself is only slightly transparent. The known apparatus therefore only provides for the inspection of bottles made of clear or transparent glass, which also have no thread in the region of the mouth, and which furthermore must not be filled with opaque liquids in this region. Moreover, internal defects in the glass are scarcely illuminated with the known apparatus, because much of the light which travels in the direction of a normal passing through the longitudinal axis of the bottle on to the surface of the bottle is lost by reflection at the surface of the bottle or at water which is present thereon or by refraction. The incident light falls in the normal direction in the known apparatus because the light source and associated camera are always diametrically opposed on opposite sides of the bottle.